Engines, for example internal combustion engines burning gasoline, diesel, or biofuel, output various substances which must be treated to meet current and future emissions legislation. Most commonly, such substances comprise hydrocarbons (HC), carbon monoxides (CO), mono-nitrogen oxides (NOx), and particulate matter such as carbon (C), a constituent of soot. Some of those substances may be reduced by careful control of the operating conditions of the engine, but usually it is necessary to provide an aftertreatment module downstream of the engine to treat at least some of the substances entrained in the exhaust gas.
Various apparatus for reducing and/or eliminating constituents in emissions are known. By these methods, engine emissions can be cleaned, meaning that a proportion of the substances which would otherwise be released into the atmosphere are instead converted to carbon dioxide (CO2), nitrogen (N2) and water (H2O).
For example, it is known to provide an oxidation device, such as a diesel oxidation catalyst (DOC), to reduce or to eliminate hydrocarbons (HC) and/or carbon monoxide (CO). Oxidation devices generally include a catalyst to convert those substances into carbon dioxide and water.
As a further example, aftertreatment modules may include filtration devices to restrict the particulates present in the exhaust gas from being output to the atmosphere. The soot collected in the filtration device must later be removed to maintain the efficiency of the filtration device. The methods by which soot may be removed from the filtration device are well known in the art and may generally be referred to as regeneration, which is carried out at elevated temperatures.
In addition, it is known to reduce or eliminate mono-nitrogen oxides (NOx) in diesel combustion emissions by selective catalytic reduction (SCR). In a typical SCR system, urea or a urea-based water solution is mixed with exhaust gas. In some applications, a urea solution is injected directly into an exhaust passage through a specialised injector device. The injected urea solution mixes with exhaust gas and breaks down to provide ammonia (NH3) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NOx) in the exhaust at a catalyst to provide nitrogen gas (N2) and water (H2O).
Such aftertreatment systems are typically modularly packaged, with conduits fluidly connecting sequential modules. It is generally desirable to provide a compact arrangement for the aftertreatment system, in order to minimise the space required for the aftertreatment system within a machine. A compact arrangement for the aftertreatment system may be facilitated by the use of flexible pipes for the conduits; however, such flexible pipework may increase the cost of the aftertreatment system. Rigid pipes are a more cost effective solution, but the use of rigid pipes to interconnect pipes of different sizes usually requires additional transitional pipework, which may increase the overall size of the aftertreatment system. It is therefore challenging to minimise the cost of the aftertreatment system whilst maintaining a compact arrangement.